A Class D audio amplifier is an electronic amplifier in which the active devices in the output stage are operated in on/off mode, i.e. as switches. The audio input signal is converted to a sequence of pulses whose average value is directly proportional to the amplitude of the signal. These types of amplifiers may use PWM (Pulse Width Modulation), pulse frequency modulation or sigma delta modulation. The frequency of the pulses is preferably much higher than the highest frequency of interest in the input signal. The switching output includes pulse trains whose width is a function of the amplitude and frequency of the signal being amplified. The output, however, typically contains undesirable spectral components (the pulse frequency and its harmonics) that may lead to distortion.
A typical Class D audio amplifier generates a PWM output signal by feeding a comparator 10 (FIG. 1) with an analog audio input 12 (FIG. 1) and an oscillator ramp input 14 from a triangular wave generator (not shown). Audio band noise present in the triangle wave is is reproduced in the square waveform PWM output signal 16 (FIG. 1). Conventional negative feedback techniques attempt to reduce the noise introduced in this stage. However, the loop gain of a class D amplifier is typically low at the high end of the audio bandwidth, thereby limiting the effectiveness of these techniques. The present invention overcomes the disadvantages of the prior art by reducing audio noise that is present on oscillator ramp input 14 (FIG. 1) instead.